A Payment API is a set of programming instructions that allows different software applications to communicate with payment processing systems. It enables businesses to accept online payments securely and efficiently by integrating payment gateways into their websites or mobile apps. This technology simplifies transactions, making it easier for customers to pay for goods and services. By using a Payment API, developers can access various payment methods, such as credit cards, digital wallets, and bank transfers. Popular providers of Payment APIs include companies like Stripe, PayPal, and Square, which offer tools to manage payments, refunds, and customer data seamlessly.